How to create a seamless low poly terrain with displace and multiple planes?
A solution can be using two additional modifiers:


Shrinkwrap to fill the gap between the two images
Data transfer to align the normal continuously




The vertex group used to that is the following:



You can also use "extend" option on the textures mapping:



Edit

Applying the data transfer modifier won't work if 'auto smooth' is not set, as Blender will recalculate the normals.
So use auto smooth (180°) and apply the modifier: